In any of various image forming apparatuses using an electrophotographic method, such as a laser printer, electrostatic copier, plain-paper facsimile apparatus, or a multi-function printer combining the above, a developing roller is used. The developing roller develops an electrostatic latent image, formed on a surface of a photoreceptor, into a toner image by means of a toner.
In order to answer various needs regarding the developing roller, the defining of its arrangement, characteristics, etc., from various viewpoints is being examined.
For example, in Patent Document 1 (Japanese Patent Application Publication No. 2008-33308), a surface treatment solution, containing an isocyanate compound, is impregnated into a surface layer portion of a conductive elastic layer, constituted by blending a conductivity imparting agent in a rubber base material, to forma predetermined surface-treated layer to perform image forming with stability. A curve formed by a Cole-Cole plot (complex impedance plot), indicating the relationship between a resistance component Zr (Ω), computed from the impedance Z (Ω) and the phase difference (θ), and a capacitive reactance component Zc (Ω), of the developing roller is thereby made to substantially have the form of a single arc. That is, an adjustment such that a single peak is exhibited is examined.
With such a developing roller, the conductive elastic layer and the surface-treated layer have the same electrical characteristics and function electrically as a single layer.
In Patent Document 2 (Japanese Patent Application Publication No. 1-252979) is examined the arranging of a developing roller by forming a conductive layer, having a polyurethane resin as a main component and a resistance value of not more than 1010 Ω·cm, on an outer peripheral surface of an elastic body layer constituted of an oil-resistant rubber with a type A hardness (JIS K6301) of not more than 40° and a resistance value of not more than 1010 Ω·cm.
It is presumed that with this arrangement, exchange of charges between elastic body layer and the conductive layer is quickened.